An oxygen sensor generally measures an oxygen concentration of a test medium, such as exhaust gases of an internal combustion engine, relative to a reference medium, such as the atmosphere. Oxygen sensors find applicability in a wide variety of areas, such as, by way of example, automotive applications, oxygen analyzers and ppO2 meters for determining oxygen concentrations of breathing gas mixtures utilized in scuba gear, soil oxygen sensors utilized in soil respiration studies, electrochemical sensors and optical sensors frequently used in marine biology or limnology oxygen measurements, and other similar types of oxygen sensors.
Oxygen sensors utilized with internal combustion engines are commonly available as either narrowband sensors or wideband sensors. A narrowband oxygen sensor comprises a single measurement cell, called a “Nernst cell,” which produces an output voltage that is dependent upon a difference in concentration of free oxygen in a test medium, such as exhaust gas of an internal combustion engine, and a reference medium such as the surrounding atmosphere. The concentration of oxygen in the atmosphere is approximately 20.8%, whereas the concentration of oxygen in exhaust gas is 0% when the engine is operating at stoichiometric conditions. Thus, the difference in oxygen concentration is 20.8%, and the narrowband Nernst cell consequently produces a voltage of approximately 450 millivolts (mV). As the difference in oxygen concentration deviates from 20.8%, however, the Nernst cell produces disproportionate output voltages that are not indicative of the actual oxygen concentration differential. Therefore, narrowband oxygen sensors are not suitable for use with internal combustion engines operating outside of stoichiometric conditions.
A wideband oxygen sensor typically comprises a Nernst cell and a pump cell enclosed within a shared volume. The pump cell is an electrochemical device which selectively pumps free oxygen within the test medium into and out of the shared volume in direct proportion to an electric current directed to the pump. During operation, a wideband oxygen sensor controller uses the pump cell to maintain a stable oxygen concentration within the shared volume such that the Nernst cell produces a steady output voltage of 450 mV, which indicates a 0% oxygen concentration in the exhaust gas. The amount of current that must be supplied to the pump cell and the output voltage of the Nernst cell indicate the actual concentration of oxygen in the exhaust gas relative to the surrounding atmosphere.
Existing wideband oxygen sensor systems utilize an analog control circuit to control the current directed to the pump cell. An inherent difficulty with such systems is that the Nernst cell and the pump cell have internal resistances that are temperature dependent. The temperature of the oxygen sensor must be maintained to keep the internal resistances of the Nernst cell and the pump cell at constant values specified by the manufacturer. Thus, a method of measuring the internal resistances within the oxygen sensor must be employed. Typically, an alternating current (AC) signal is directed through a resistor having a known value, and the resultant attenuation of the AC signal is measured to determine the internal resistance of the Nernst cell. Although the AC signal approach leads to reasonably accurate measurements, the AC signal has a tendency to create oscillations within the analog control circuit, causing it to become unstable. Consequently, the Nernst cell signal must be processed by a low-pass filtering circuit to remove the AC signal and enable the analog feedback loop to operate properly.
A drawback to filtering the Nernst cell signal is that it slows down the entire system. This slowing down of the system is so severe that the system cannot resolve transitioning concentrations of oxygen in exhaust gases, such as during vehicle acceleration, deceleration, and other engine-load changes. While the wideband oxygen sensor is perfectly capable of resolving transitioning concentrations of oxygen, the analog control circuit and filtering circuitry renders the sensor useless except under steady states of engine operation. What is needed, therefore, is a discrete-time, digital controller for an oxygen sensor which accurately resolves oxygen concentrations of exhaust gases during transitioning concentration states, as well as steady states.